1. Field of the Invention
The invention concerns aluminum sheet alloys of the 6000 series according to the nomenclature of The Aluminum Association, that are clad with another aluminum alloy and intended for the manufacture of aircraft structural parts, especially airplane fuselage panelling.
2. Description of Related Art
The fuselages of civil aircraft are for the most part made from 2024 alloy sheet, clad on either surface with a low composition aluminum alloy, an 1050 or 1070 alloy for example. The thickness of the cladding on each surface may, according. to the thickness of the core sheet, typically represent between 1% and 15% of the total thickness.
The purpose of the cladding alloy is firstly to provide sufficient corrosion resistance. Slightly generalized or pitting corrosion is tolerable but it must not be penetrating so as not to attack the core alloy. This evaluation must be made on the clad sheet in the temper of use, that is to say typically after solution heat treatment, quenching and optionally artificial ageing, since diffusion phenomena between core and cladding at the time of solution heat treatment may deteriorate the corrosion resistance of the cladding.
The cladding must also be able to act as a cathodic protection vis-à-vis the core, that is to say a sufficient difference in electrochemical potential must exist between cladding and core. This protective effect comes into play in particular should a scratch reach the core. The cladding may have other functions such as physical protection of the alloy against scratches and impacts, the providing of a layer that is easy to polish, for airlines desiring a metal rather than a painted appearance, or improved ability to undergo surface treatment, chemical polishing in particular, and the various pre-treatments that are required for the application of polymer coatings.
For several years, in lieu and stead of the 2024 or related alloys, AlMgSi alloys of the 6000 series have been proposed for fuselage panels. These alloys, which are also heat treatable, offer good mechanical characteristics in the heat treated state, a high elastic modulus and lower density than the 2024. In general, they have better corrosion resistance than the 2024 alloy. The alloys in the 6000 series are readily weldable making it possible to reduce the number of riveted assemblies which are a source of corrosion. Some of these alloys may be used in unclad form, in particular in metallurgic tempers that have been specially desensitized to intergranular corrosion. Such heat treatment process is described in patent EP 0787217, on behalf of the applicant, which concerns a special ageing treatment for 6056-type alloys, such as to desensitize the alloy to intergranular corrosion and allow its use for aircraft fuselages. Nevertheless, even in this temper, the 6056 alloy may show certain sensitivity to pitting corrosion. Preference may therefore be given to clad sheet.
U.S. Pat. No. 4,589,932 (Alcoa) describes the use for airplane structural parts of an alloy later registered under designation 6013, having the composition (% by weight): Si: 0.4-1.2, Mg: 0.5-1.3, Cu: 0.6-1.1, Mn: 0.1-1, Fe less than 0.6.
Patent EP 0173632, filed on behalf of the applicant, describes an alloy later registered under designation 6056, having the composition (% by weight): Si: 0.9-1.2, Mg: 0.7-1.1, Cu: 0.3-1.1, Mn: 0.25-0.75, Zn: 0.1-0.7, Zr: 0.07-0.2, Fe less than 0.3.
Patent FR 2704557, also filed on behalf of the applicant, concerns clad sheet for fuselages comprising a core in alloy 2000 or alloy 6000 containing copper, and cladding whose difference in potential relative to the core is between 90 and 130 mV. The cladding alloy is preferably a 6000 alloy having the composition (% by weight): Si: 0.15-0.6, Mg: 0.3-0.8, Cu less than 0.1 Mn less than 0.4, Fe less than 0.4.
The manual  less than  less than Aluminumxe2x80x94Properties and Physical Metallurgy greater than  greater than  published by the American Society for Metals under the direction of J. E. Hatch (1984), pp. 301, 372 and 373, gives as clad products ( less than  less than Alclad greater than  greater than ) products which for cladding use alloys of the 1000 type or the 7072 alloy (AlZn alloy containing approximately 1% Zn) for cores in 2000 alloys; the 7072 alloy is also used for cores in alloys 3003, 3004, 6061, 7075 and 7178. According to standard ASTM B209, cladding alloys may also include alloys in the 6000 series (2014 clad with 6003) and other alloys of 7000 type (such as 7008 and 7011).
As a general rule, the above-mentioned manual by Hatch recommends a difference in potential between the cladding and the core of at least 100 mV. Products do exist however which have a lower potential than 100 mV, such as Alclad 2014 (2014 T8 clad with 6003)and 2014 T8 clad with 1050. Moreover, experience of persons skilled in the art shows that it is preferable to limit the difference in potential to a value of approximately 130 mV to 150 mV, in order to avoid too high corrosion risks when assembling the two layers of metal. It is therefore considered that a difference in potential of between approximately 60 mV and 130 mV provides the clad product with sufficient cathodic protection.
The purpose of the present invention is to provide sheet alloy for fuselages in an alloy of the 6000 series, with which it is possible to attain properties of use that are comparable with those of the 2024 alloy, whose cladding ensures sufficient corrosion resistance in the heat treated state and sufficient cathodic protection, which does not however go as far as causing fast disappearance of the cladding layer, which would be contrary to the desired protection. It is also desirable that the cladding alloy should be chosen such that its application to the core alloy by co-rolling can be achieved without too much difficulty, that it does not contain elements that hinder the recycling of production scrap and that it has acceptable mechanical resistance so as not to deteriorate the mechanical resistance of the clad sheet.
A further purpose of the present invention is to put forward a process for manufacturing an aircraft structural part incorporating said clad sheet with which it is possible to draw maximum benefit from the metallurgical characteristics of this clad sheet.
The subject of the invention is sheet clad on one or two surfaces, made up of a core sheet in an alloy having the composition (% by weight): Si: 0.7-1.3, Mg: 0.6-1.2 (preferably 0.7-1.1), Cu: 0.5-1.1, Mn: 0.15-1.0 (preferably 0.3-0.7), Zn less than 0.5, Fe less than 0.5 (preferably  less than 0.3), Zr less than 0.2, Cr less than 0.25, other elements  less than 0.05 each and  less than 0.15 total, the remainder aluminum, and of cladding in an aluminum alloy on at least one surface of the sheet, having a thickness per surface of between 1 and 15% (and preferably 1.5 to 12%) of the total thickness, this cladding alloy being an alloy of AlZn type containing (% by weight) from 0.25 to 0.7% (and preferably from 0.3 to 0.6%) of zinc, less than 0.40% iron, less than 0.40% (and preferably less than 0.25%) silicon, less than 0.10% of each of the elements copper, manganese, magnesium, titanium or vanadium, and less than 0.05% each and 0.15% total for the other elements, the remainder aluminum.
A further subject of the invention is an aircraft structural part, such as a fuselage coating panel, comprising sheet clad on one or both surfaces made up of a core sheet and cladding, the core sheet and the cladding having the above-mentioned compositions.
A further subject of the invention is a manufacturing process to produce an aircraft structural part comprising:
a) the manufacture by hot rolling of clad sheet, made up of a core sheet in an alloy having the composition (% by weight): Si: 0.7-1.3, Mg: 0.6-1.2 (preferably 0.7-1.1) Cu: 0.5-1.1, Mn: 0.15-1.0 (preferably 0.3-0.7) Zn less than 0.5 Fe less than 0.5 (and preferably  less than 0.3) Zr less than 0.2 Cr less than 0.25 other elements  less than 0.05 each and  less than 0.15 total, the remainder aluminum, and of cladding in aluminum alloy on at least one surface, having a thickness per surface of between 1 and 15% (and preferably 1.5 to 12%) of the total thickness, this cladding consisting on at least one surface of an AlZn alloy containing (% by weight) from 0.25 to 0.7% (and preferably 0.3 to 0.6,%) zinc, less than 0.40% iron, less than 0.40% (preferably less than 0.25%) silicon, less than 0.10% of each of the elements copper, manganese, magnesium, titanium or vanadium, other elements  less than 0.05% each and  less than 0.15% total, the remainder aluminum, the other surface possibly being clad with a second alloy, in particular an alloy of 4000 type;
b) optionally one or more cold rolling passes;
c) solution heat treatment followed by quenching;
d) optionally one or more finishing operations such as crease recovery, roller levelling or controlled stretching;
e) optional sheet forming;
f) assembly of the structural part by riveting several sheets, and welding or riveting on stiffeners or frames.
g) optional artificial ageing of the structural part.